CAM4 Uniform Aerosol Simulation Issue: Modern Topography Effects on AEROD_v and FSDS

[Yue]

Hello everyone,

I am using CESM1.2 with the CAM4 component for a paleoclimate simulation with an ancient topography. I have set up globally uniform aerosol forcing by modifying the input files for prescribed aerosols (prescribed_aero_file) and aerosol deposition fluxes (aerodep_flx_file) to be homogeneous, without any spatial variation. The goal is to have aerosol concentrations at 1500 times the modern global mean.

However, the output shows inconsistencies: the AEROD_v (total aerosol optical depth in the visible band) and surface solar radiation fluxes (FSDS and FSDSC) still reflect patterns influenced by modern topography. For example, in regions where the ancient topography is ocean but the modern topography is land, FSDS values are unusually low, and AEROD_v is abnormally high. The land fraction output (landfrac) correctly matches the ancient topography, indicating that the base topography is applied properly.

I have already verified that the input aerosol files are globally uniform. Could you please advise which files or processes in CAM4 might be causing this issue? What steps should I take to ensure that AEROD_v and FSDS also become globally uniform, matching the homogeneous aerosol inputs?

Thank you for your assistance.

 

[Danny Leung]

Hi Yue--
The model version is a bit old and I don't think the CAM-Chem people supports CESM1/CAM4 anymore...
To just brainstorm a bit, I feel like the land sees your topography but I am not sure if CAM sees it. Maybe CAM completely doesn't see it, or maybe only the radiative transfer module does not see it (Are you using mam/rrtmg or bam/carmt?)
To check if CAM4 sees it at all, maybe you can check some dynamical variables in CAM4? E.g., aerodynamic resistance (RAM1?), 10-m wind speed (U10), and friction velocity (airFV?) change drastically from land to ocean, and you can sort of see the shape of the continents by plotting their global maps.
Another thought is that you might have too much vegetation in your paleo case, so the continents are as dark/non-reflective as the oceans. Just brainstorming...
Good luck,
Danny

 

[Yue]

Hi Danny,
Thanks for your suggestions! I've identified the issue – it was due to an internal PS variable setting. After adjusting it, the simulation is now working correctly.I appreciate your help in brainstorming this. Your insights were very helpful in guiding my troubleshooting.
Best regards,
Yue